1 Technical Field
The invention relates generally to a gas sensor engineered to measure exhaust gas that is a target to be measured.
2 Background Art
Gas sensors designed to measure exhaust gas emitted from an internal combustion engine typically include a sensor device, a housing, a lead cover, a contact-spring insulator, and contact springs. The sensor device work to measure the exhaust gas. The housing retains the sensor deice using a sensor insulator. The lead cover is secured to the housing. The contact-spring insulator retains contact springs therein. The spring contacts are placed in contact with electrical leads of electrodes affixed to the sensor device and electrical leads of a heater arranged on the sensor device. The spring contacts are connected through connecting terminals to electrical leads wired in an external control device located outside the gas sensor.
The lead cover often has formed therein inlet holes through which air used as a reference in measuring the gas in the sensor device is introduced from outside the gas sensor. A filter is arranged on the inlet holes which admits gas to pass therethrough, but blocks liquid. The air, having entered the head cover at the inlet holes with the filter, passes through a hole of the contact-spring insulator in which the contact springs are disposed and reaches the sensor device. Japanese Patent First Publication No. 2013-178228 discloses such a type of gas sensor.
Usually, conditions where gas sensors which measure the exhaust gas emitted from the internal combustion engine are used undergo a wide change in temperature resulting from a cyclic change in temperature of the internal combustion engine. Liquid water usually passes through the filter on the inlet holes of the lead cover, but water vapor, as produced from evaporation of the water at high temperatures, will pass through the filter and then enter the lead cover.
Gas sensors which are not equipped with the above type of a filter disposed on the inlet holes also have a risk that water vapor enters the lead cover. For instance, gas sensors with no inlet hole have a sealing mechanism which blocks the entrance of the exhaust gas also face a risk that the sealing mechanism cannot hermetically seal the lead cover completely, so that the exhaust gas enters the lead cover. Usually, electrical leads do not have a complete sealing structure and, thus, encounter a risk that air enters the lead cover.
The gas sensors which measure the exhaust gas are usually exposed to high temperatures and thus engineered to work at a temperature (e.g., hundreds of degrees) higher than a dew point at which water vapor will start being condensed. Usually, water vapor, having entered the lead cover, is drained without being condensed along the same path as when it enters the lead cover. A change in temperature of the gas sensor arising from splashing with rain, however, may cause the water vapor in the lead cover to become liquid.
The amount of the condensed water entering the lead cover is usually small. When the temperature temporarily has dropped and then returned back to a normal level, the condensed water in the lead cover will be evaporated again, so that the water vapor is discharged from the lead cover. The above publication, therefore, does not consider dew condensation.
However, in order to deal with exhaust emission regulations which will be more tightened, there is a need for eliminating electrical noise added to an output of the gas sensors to improve the accuracy of the sensor output. For example, in a case where NOx gas which is infinitesimally contained in the exhaust gas is measured by a gas sensor, an output current from the gas sensor is usually very small. Such a type of gas sensor, therefore, needs to minimize adverse effects of noise on the output of the gas sensor for enhancing the measurement accuracy thereof.
The gas sensor designed to measure NOx gas usually measures electrical current flowing between two electrodes disposed on a sensor device installed in the gas sensor. Therefore, when the condensed water simultaneously touches a contact spring and a connecting terminal electrically connected to one of the electrodes and a contact spring and a connecting terminal electrically connected to the other electrode, it will cause a small amount of leakage current to flow among them, which may generate noise impinging on the output of the gas sensor.